1. Technical Field
The present invention relates to membranous-vibration sound absorbing materials which feature excellent sound-absorbing characteristics, lightweight and excellent workability, as well as transparency so that people can see therethrough if necessary, and dust-proof and dust-free properties, and processes for producing the same.
2. Background Art
A host of sound absorbing materials are used to control noise in various locations such as airports, expressways and their surroundings, music halls, gymnasiums, construction sites, tunnels and clean rooms.
The sound absorbing materials are all made of porous materials which includes soft porous materials such as glass fiber, rock wool and sponge, porous molded boards such as porous sintered boards, metal fiber boards and foamed metal boards. Therefore, there is the general concept that sound absorbing materials are porous materials.
These sound absorbing materials have their own merits and demerits in terms of sound-absorbing characteristics, weatherability, cost and fabrication method and suitable types are used in accordance with the specific use of interest.
In recent years, the sound absorbing materials have been used for various applications and various demands are made for characteristics other than sound-absorbing characteristics.
One of the characteristics is that the other side can be seen through the sound absorbing material at a location where the sound absorbing material is installed, that is, transparency.
For instance, a sound-proof wall made of a sound absorbing material is installed as a side wall on both sides of a road to prevent noise pollution to residents along the road.
Polycarbonate has been used in insulation walls on expressways because of its good sound insulating property, but it has the demerit that the level of internal reflected sound is high. Moreover, such a conventional sound-proof wall perfectly shuts out the view of scenery outside the road so that passengers cannot see outside scenery and brings a sense of oppression to drivers. Therefore, the advent of a sound absorbing material having sound-absorbing characteristics as well as transparency is desired.
Noises at factories and research facilities include ones generated from various types of machine tools, measuring instruments, pumps, boilers, power generators and the like.
These noise sources are operated under a condition surrounded by sound absorbing materials made of porous materials such as glass fiber if necessary. Since the conventional sound absorbing materials do not have transparency, it is impossible to check the operation condition of machines through the sound absorbing materials. Therefore, the advent of a sound absorbing material having transparency is also desired in these places.
Meanwhile, it is necessary to perfectly prevent generation and entry of dust in clean rooms and the like. However, the conventional sound absorbing materials are not suitable for use in clean rooms because they are made of porous materials.
The present invention is intended to solve the above mentioned problems of the prior art, and therefore, the principal object of the present invention is to provide membranous-vibration sound absorbing materials which ensures not only excellent sound absorbing characteristics, workability and strength but also transparency that people can see therethrough and dust-proof and dust-free properties if necessary, and which can be used suitably for such applications as clean rooms, as well as a process for producing the same.
In order to attain the aforementioned object, the present inventors conducted intensive studies and accomplished the present invention on the basis of the following observations.
As already mentioned, most of the commercial sound absorbing materials used today are made of soft porous materials such as glass fiber and rock wool; porous molded boards such as porous sintered boards, metal fiber boards and metal foamed boards; and perforated plates such as perforated metal sheets.
As a result of studies on these sound absorbing materials, the inventors found it difficult to ensure transparency for these sound absorbing materials.
Meanwhile, it is known that sound is slightly absorbed by the membranous vibrations of a thin film such as a vinyl sheet, that is, causing vibrations in a smooth plane.
However, the sound absorbing effect achieved by utilizing the vibration of a smooth plane is very low and the maximum attainable absorption coefficient has been on the order of 0.3-0.5, which is too low to realize the commercial application of this concept.
As a result of the intensive studies on sound absorption by the membranous vibrations of a thin film, the inventors made the following discovery.
As one of sound absorbing materials used conventionally, widely known is a sound absorbing material 54 made of glass fiber 50 whose surface is covered with a thin film 52 such as a vinyl sheet as a surface protective material as shown in FIG. 1(a) and FIG. 1(b). Such a sound absorbing material 54 is generally combined with a sound insulating material 56 (a rigid body), but its sound absorbing characteristics vary depending on the state of the thin film 52.
In a state shown in FIG. 1(a), that is, a state that the thin film 52 is loosely stretched, the characteristics of the sound absorbing material 54 are mainly determined by the density, thickness and the like of the glass fiber 50. In addition to these, the thickness, rigidity and the like of the thin film 52 are controlled to achieve a sound absorbing material which exhibits excellent sound absorbing characteristics over a broad frequency range.
However, if the area of the thin film 52 is small, as shown in FIG. 1(b), the characteristics of the sound absorbing material 54 are determined by the tensile strength, thickness and the like of the thin film 52 irrespective of the density and thickness of the glass fiber 50 when the sound absorbing material 54 is constructed by covering the glass fiber 50 with the thin film 52 having tension.
In other words, under a condition that the thin film has flexibility like the sound absorbing material shown in FIG. 1(a), when sound waves G hit the thin film 52, the absorption coefficient is determined by the characteristics of the glass fiber 50 in most cases.
Contrary to this, under a condition that the thin film 52 has tension as shown in FIG. 1(b), most of the sound waves G are reflected by the thin film 52, and sound absorbing characteristics are almost determined by the thin film 52. Here, the sound absorbing characteristics of the thin film 52 are determined by a resonance frequency f.
The resonance frequency f is represented by the following formula: EQU f=[C.sup.2 (.rho.m/E).sup. 1/2 ]/1.8t
where C represents acoustic velocity, .rho.m the volume density of the thin film 52, t the thickness of the thin film 52 and E the Young's modulus of the thin film 52.
As being obvious from the above formula, when the volume density .rho.m is constant, the resonance frequency f is greatly affected by the Young's modulus of the thin film 52, that is, the tension and thickness t of the thin film 52.
In this instance, such a thin film does not keep its own shape. For this reason, in order to make the thin film usable as a sound absorbing material, it is necessary to hold the thin film 52 with some holding member while the thin film keeps appropriate tension as described in the foregoing.
As a result of studies on a method to hold the thin film suitably under a condition that appropriate tension and transparency are ensured, the present inventors found that a thin film having a continuous plane or a plurality of ruptures is sandwiched between sheets having a large number of openings such as expanded metal sheets, perforated metal sheets and metal screens to hold the thin film ensuring its tension and transparency, and that the sound absorbing effect of the aforementioned membranous vibrations is combined with the sound absorbing effect of the sheets to achieve extremely excellent performance in sound absorbing characteristics. The inventors thus accomplished the present invention based on this discovery.
In other words, according to the first aspect of the present invention, there is provided a membranous-vibration sound absorbing material in which a resin thin film is held between sheets having a large number of openings.
According to the second aspect of the present invention, there is provided a membranous-vibration sound absorbing material including a porous material and a laminate in which a resin thin film is held between sheets having a large number of openings.
In the above-mentioned membranous-vibration sound absorbing material of the present invention, the sheets sandwiching the resin thin film preferably have openings of different sizes.
In the above-mentioned membranous-vibration sound absorbing material of the present invention, the sheets are preferably ones selected from expanded metal sheets, perforated metal sheets and metal screens.
In the above-mentioned membranous-vibration sound absorbing material of the present invention, the sheets are preferably made of aluminum and/or iron.
When the above-mentioned membranous-vibration sound absorbing material of the present invention is used as an insulation wall on expressways and the like, the resin thin film is preferably transparent.
Moreover, when the above-mentioned membranous-vibration sound absorbing material of the present invention is used as a sound-proof wall in clean rooms and the like, the resin thin film preferably has a continuous plane.
Furthermore, when the above-mentioned membranous-vibration sound absorbing material of the present invention is used as a sound-proof wall in locations other than clean rooms, the resin thin film preferably has ruptures.
These membranous-vibration sound absorbing materials of the present invention can be adapted to various modes according to application purpose.
For instance, the sound absorbing material which uses a resin thin film having a continuous plane entirely and no ruptures at all to ensure not only sound absorbing characteristics but also extremely excellent dust-proof and dust-free performances can be used for such application as clean rooms.
Moreover, it is possible to achieve better sound absorbing characteristics by attaching a porous material to the rear side of the sound absorbing material (a side of the sound absorbing material opposite to sound sources) while dust-proof and dust-free performances for clean rooms are maintained.
Meanwhile, since good transparency is ensured by using a transparent resin thin film, the sound absorbing material of the present invention can be used suitably for such applications requiring transparency as a noise controlling absorbent for machines in factories and an insulation wall on airports and expressways.
In addition, better sound absorbing characteristics can be attained by making ruptures in the resin thin film to create membranous vibrations. Therefore, the sound absorbing material of the present invention can achieve extremely excellent performance in sound absorbing characteristics under a condition that transparency is ensured. As a result, the sound absorbing material can be suitably used especially for applications requiring both transparency and excellent sound absorbing characteristics such as expressways and factories as described above. In this case, the sound absorbing material may be used in combination with a transparent sound insulating material.